1. Field of the Invention
This invention relates to a cryogenic device that uses a liquefied gas refrigerant such as liquid helium and liquid nitrogen, and more specifically to a refrigerant liquid level measuring device and a refrigerant liquid level measuring method for measuring the liquid level of a liquefied gas refrigerant and a superconducting magnet device including the device.
2. Description of the Related Art
In a superconducting magnet device as a cryogenic device in general, its superconducting coil must be maintained in a superconductive state. In a superconducting magnet device of a refrigerant liquid immersion type such as those immersed in liquid helium, the superconducting coil is immersed in the liquid helium and maintained at extremely low temperatures, so that the superconductive state is maintained. The superconducting magnet device keeps the liquid helium in a cryogenic container (cryostat) and prevents the liquid helium from being evaporated. However, it is impossible to completely prevent heat penetration from the outside, and therefore the liquid helium is gradually evaporated. Even in a superconducting magnet device provided with a refrigerator for condensing gas helium in the cryostat into liquid helium, the liquid helium is gradually evaporated and reduced as the magnet is excited and demagnetized. If the liquid helium is reduced, the cooling effect upon the superconducting coil becomes insufficient, which is likely to destroy the superconductive state and can give rise to quenching of the superconducting magnet. It is therefore critical to monitor the amount of the liquid helium in order to operate the superconducting magnet device, and most superconducting magnet devices include a helium liquid level measuring device.
The helium liquid level measuring device has the following structure. The helium liquid level measuring device includes a filament (superconducting wire) of a superconducting material such as a niobium-titanium alloy provided in the vertical direction so that the filament crosses the liquid level of the liquid helium in a cryostat. A normal conductor heater made for example of a manganese wire is electrically connected in series with the upper part of the superconducting filament, and the superconducting filament is thermally contacted to the heater to form a sensor element of the superconducting filament. When constant current as shown in FIG. 8 is passed through the sensor element of the superconducting filament, the heater generates Joule heat by the current, so that the superconducting filament above the liquid level of the helium attains a normal conductive state. The superconducting filament in the normal conductive part has electrical resistance, but the superconducting filament below the liquid level of the helium is kept in the superconductive state and the electrical resistance of the part is zero. The electrical resistance value of the superconducting filament at the time consists only of the resistance value of the part of the superconducting filament in the normal conductive state and changes depending on the length of the part of the superconducting filament above the liquid level. Therefore, voltage generated across both ends of the superconducting filament is measured using a voltmeter, and the resistance value is measured based on the passed current, so that the liquid level (the height of the liquid surface) can be obtained. Note that a similar liquid level measuring device is disclosed by International Publication No. 97/08518 pamphlet (Abstract and FIG. 1) or JP-UM-A-62-141721 (Claim for Utility Model Registration and FIG. 2).
However, according to the conventional method of measuring the liquid level of helium using the refrigerant liquid level measuring device using the superconducting wire, (1) the length of the superconducting wire that must attain a normal conductive state increases as the length of the refrigerant liquid level measuring device increases, and (2) the degree of the thermal contact between the heater and the superconducting wire varies within the range of working accuracy. For these reasons, it takes long before the boundary between the part of the superconducting wire in the superconductive state and the part in the normal conductive state to be lowered to reach the liquid level of the helium. The necessary time is greatly affected by individual differences in the degree of the thermal contact between the heater and the superconducting wire in the refrigerant liquid level measuring device. Therefore, using refrigerant liquid level measuring devices identical in design and structure, the time required for the boundary between the parts of the superconducting wire in the superconductive state and the normal conductive state to reach the liquid level of the helium significantly differs because of the individual differences, and therefore the liquid level cannot accurately be measured simply by passing uniform current for a uniform time period.